[Storebror]

The latest patched version is 4.13.4, just saying, but it won't help much either.
This is a glitch which is buried deep inside the game's code.
The online replication code doesn't replicate trigger presses quick enough.
Sometimes the "weapon bit mask" replication comes first, and if a bomb is set to zero in that mask, but it's trigger keypress has not been replicated yet, the bomb will just be removed on all remote clients.

It will fall, impact, inflict damage etc., but the other players just won't see what the player sees who dropped the bomb.

We've been deeply investigating this issue and finally solved it with Ultrapack 3 Patch Pack 210, but I don't know of any other modpack having solved this and no base game version ever changed anything concerning this issue.

Before someone starts yelling "but I saw a drop falling off my wingman's belly" - Yes, this issue doesn't always happen, it's a race condition between net replication of weapon triggers and net replication of occupied weapon stores.
One (triggers) happens only every 4 (or so) ticks and might be skipped if connections are laggy, the others is guaranteed to happen with each and every tick.
An issue "by design".

Best regards - Mike